Xenobiotic-metabolizing enzymes are responsible for metabolism and inactivation of all clinically used drugs. They are also involved in the metabolic activation or inactivation of toxins, mutagens and chemical carcinogens. Marked differences in levels of expression of these enzymes have been found in humans and these differences could contribute to interindividual differences in sensitivities to drugs and carcinogens. Variable gene expression could account for some differences in levels of expression of xenobiotic-metabolizing enzymes. Most of these enzymes are expressed in the liver and their genes are under control of different hepatocyte-enriched transcription factors. Several families of transcription factors are preferentially expressed in the liver and control liver-specific gene expression. Typically, in vitro techniques, including transfections of reporter gene constructs and protein-DNA binding assays, are used to study gene regulation. However, it is difficult to directly demonstrate that the results obtained using in vitro studies actually reflect gene expression in the intact animal. Studies to determine whether hepatocyte-enriched factors are involved in regulating gene expression in vivo can be done by using gene knockouts to disrupt expression of transcription factors and then determine the effects of transcription factor loss on target gene expression. To investigate the role of liver-enriched transcription factors in control of P450 gene expression and expression of other genes involved in liver function, null mice are being produced. In particular, conditional gene disruption is required since embryonic disruption of transcription factor genes frequently results in embryonic lethality or early neonatal death. Conditional-null mice produced using the Cre-loxP method were developed for the transcription factors HNF-1alpha, HNF-4alpha and C/EBPalpha. Phenotypes are observed and gene expression patterns determined using Northern blot and Western blot analyses. The data indicate that mice lacking expression of these transcription factors develop severe phenotypes including diabetes, dwarfism, hyperbilirubinemia, hypercholestemia and hypolipidemia. Recent studies are using an novel tamoxifen-actvating Cre to generate temporal-specific gene knockout models to study compensatory mechanisms of gene control in liver.